From the DE-PS 750 957 there is known a process for improving the corrosion resistance of metals, in particular of iron and steel, by treating them in a solution forming phosphate coatings, where the solution contains an accelerator, and where nitromethane, nitrobenzene, picric acid, a nitroaniline, a nitrophenol, a nitrobenzoic acid, a nitroresorcinol, nitrourea, a nitrourethane or nitroguanidine is used as accelerator. The optimum concentration for the individual accelerators is different, but in the phosphatizing solutions it is generally in the range between 0.01 and 0.4 wt-%. For the accelerator nitroguanidine the optimum concentration is said to be 0.2 wt-%. From the DE-OS 38 00 835 there is known a process of phosphatizing metal surfaces, in particular surfaces of iron, steel, zinc and the alloys thereof as well as aluminum, as a pretreatment for the cold working, where without activation at a temperature in the range from 30 to 70.degree. C. the surface is brought in contact with an aqueous solution containing 10 to 40 g Ca.sup.2+ /l, 20 to 40 g Zn.sup.2+ /l, 10 to 100 g PO.sub.4.sup.3- /l and as accelerator 10 to 100 g NO.sub.3.sup.- /l and/or 0.1 to 2.0 g organic nitro compounds per liter, where the solution has a pH value in the range from 2.0 to 3.8 and a ratio of free acid to total acid of 1:4 to 1:100. As accelerator, an m-nitrobenzene-sulfonate and/or nitroguanidine may be used. The phosphate coatings produced in accordance with the known process have coating weights of 3 to 9 g/m.sup.2.
Although it is known per se that nitroguanidine can be used as accelerator when phosphatizing metallic surfaces, the practical use of this accelerator meets with some difficulties, as the phosphatizing results achieved are very frequently unsatisfactory. This is quite obviously due to the fact that the effect of the accelerator nitroguanidine very much depends on the inorganic components of the phosphatizing solution and the concentration of the inorganic components of the phosphatizing solution, so that the phosphate coatings produced by using nitroguanidine only have good functional properties when one succeeds in providing a phosphatizing solution in which the individual components are adjusted to each other such that when using nitroguanidine as accelerator, phosphate coatings of good, constant quality can be produced also in a continuous operation. Moreover, the interaction between the nitroguanidine and the remaining components of the phosphatizing solution cannot be predicted or determined by theoretical considerations or simple experiments, but must be determined by extensive experiments on different phosphatizing systems. The frequently unsatisfactory results are also due to the poor water solubility and the uneven distribution of the nitroguanidine.